System for increasing the shock loading resistance of structures

ABSTRACT

A reinforced underground structure and a method for constructing the same. A below ground chamber or excavation is surrounded and defined by a concrete support wall. A frozen earthen formation surrounds the concrete wall. Preferably, the frozen zone includes an inner frozen zone which is adjacent the concrete wall, an outer frozen zone which is spaced from the inner frozen zone, and an intermediate confined zone is frozen under a preselected internal compressive stress so as to increase the overal strength of the structure in resistance particularly to external shock loads.

United States Patent 1 Anderson et al.

[451 Feb. 6, 1973 1 1 SYSTEM FOR INCREASING THE SHOCK LOADING RESISTANCE OF STRUCTURES [75] Inventors: Philip J. Anderson, Deerfield; Daniel Y. C. Ng, Chicago, both of [73] Assignee: Institute of Gas Technology 221 Filed: Dec. 1, 1970 [21] App1.No.: 93,981

[52] U.S.C1 ..109/l S,61/.5,61/36A [51] Int. Cl. ..E04h 9/12, B65g 5/00 [58] Field of Search ..6l/.5, 36 A, 34; 109/1 S [561 References Cited UNITED STATES PATENTS 2,437,909 3/1948 Cooper "61/.5 3,354,654 11/1967 Vignovich ..61/.5

3,151,416 10/1964 Eakin .,6l/.5 3,295,328 1/1967 Bishop 3,41 1,302 11/1968 Nachsen ..61/.5

Primary ExaminerReinaldo P. Machado Attorney-Molinare, Allegretti, Newitt & Witcoff [57] ABSTRACT A reinforced underground structure and a method for constructingthe same. A below ground chamber or excavation is surrounded and defined by a concrete support wall. A frozen earthen formation surrounds the concrete wall. Preferably, the frozen zone includes an inner frozen zone which is adjacent the concrete wall, an outer frozen zone which is spaced from the inner frozen zone, and an intermediate confined zone is frozen under a preselected internal compressive stress so as to increase the overal strength of the structure in resistance particularly to external shock loads.

12 Claims, 2 Drawing Figures SYSTEM FOR INCREASING THE SHOCK LOADING RESISTANCE OF STRUCTURES BACKGROUND OF THE INVENTION FIELD OF THE INVENTION AND DESCRIPTION OF THE PRIOR ART This invention relates to a reinforced belowground structure and to a method for constructing the same.

There are, in existence, reinforced belowground or 1 underground chambers or excavations used for various purposes. Such structures, for example, are used for storing valuable records or materials, while other types of structures are used for various defense facilities, such as bomb shelters. Generally, such facilities are constructed by the use of steel plates, reinforced concrete supporting structures, or a combination of steel and reinforced concrete supporting structures. In the case of high strength steel construction, the expenses and construction difficulties generally make such structures unfeasible. In the case of reinforced concrete structures, even with the steel reinforcement, this type of structure is not capable of withstanding high stresses, which might result from shock loading in proximate locations. The purpose of these underground structures often is to withstand compressive shock waves created by an impact in the vicinity of the structure. The impact may be created in any conceivable manner involving the release of a specified amount of energy over a selected period of time.

The fragmentation of a structure or earth formation upon the passage of such a shock wave is a rather complex mechanism. At the point of the impact or release of energy, a crushed zone or crater is formed with a central impact zone. The limits of the crushed zone or crater are defined as that volume wherein the compressive-shear stress exceeds the compressive-shear strength of the material which has been subjected to the impact. Beyond the crater zone itself, shock wave compressive-shear pulse or wave radiates outwardly through the surrounding area in all directions. Thus, in the case of an impact within a structure, the shock wave is substantially hemispherical.

As the shock wave moves through the surrounding area, the intensity of the shock wave is decreased. However, even though the intensity of the shock wave is decreased below the compressive strength of the material through which it passes, the passage of the shock wave produces fractures which radiate or extend radially from the impact zone or crater, resulting from a tangential tensile stress. The tensile strength of the surrounding area in resisting this tensile stress controls the degree of fracturing thereof.

At a given free surface within the formation surrounding the impact zone, the compressive shock wave passing therethrough imparts a tensile stress or force to the material. This tensile force produces a slabbing or fracturing of the free surface. Thus, the tensile strength again controls the amount of fracturing. In order to resist such fragmentation processes, it is necessary to resist the compressive-shear stresses in the crater zone, the transverse tensile stress in the radiating zone, and the reflected tensile stress in the free surface region. As described previously, the prior art structures are deficient in one or more respects in their ability to adequately resist this complex impact force.

SUMMARY OF THE INVENTION It is therefore an important object of this invention to provide a reinforced below ground structure for resisting shock waves and a method for constructing the same wherein disadvantages of prior art structures are substantially avoided.

It is also an object of this invention to provide a belowground reinforced structure or excavation which is 0 capable of resisting the complex compressive and tensile stresses created as a shock wave passes through the zone or area in which the structure is located.

It is a further object of the invention to provide a reinforced belowground structure and a method of constructing the same wherein the structure resists the compressive-shear stress created in the impact zone, the transverse tensile stress in the radiating zone, and the reflected tensile stress in the free surface region.

It is yet another object of this invention to provide a belowground reinforced structure which is reinforced by a concrete wall which is surrounded by a frozen zone, the concrete wall and the frozen zone combining to provide a particularly effective reinforcement for the belowground structure for the purpose of resisting shock wave stresses.

It is yet another object of this invention to provide a reinforced belowground structure wherein the tensile stress produced by fragmentation of the zone around a crater is effectively countered by the combination of a reinforced concrete wall surrounded by a zone frozen under a preselected compressive stress.

Further purposes and objects of this invention will appear as the specification proceeds.

The foregoing objects are accomplished by providing a belowground structure including a belowground chamber, a concrete wall surrounding the chamber, and a frozen earthen formation surrounding the concrete wall, the concrete wall and the frozen earthen formation cooperating to provide a reinforced zone surrounding said chamber having an enhanced structural strength for resisting stresses created by an impact in a relatively close region. Preferably, the frozen zone includes an inner frozen zone adjacent the concrete wall, an outer frozen zone spaced from the inner frozen zone, and an intermediate zone frozen under a preselected internal compressive stress.

BRIEF DESCRIPTION OF THE DRAWINGS Particular embodiments of the present invention are illustrated in the accompanying drawings wherein:

FIG. I is a schematic, vertical cross-sectional view illustrating one preferred form of our invention; and

FIG. 2 is another schematic vertical cross-sectional view through an alternate and preferred embodiment of our invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring first to the embodiment of FIG. 1, our invention is shown in its most simplified form. An excavation of suitable width and depth is first made in the earth. A concrete, preferably steel reinforced, structure 10 is then constructed in the excavation, using usual techniques. The concrete structure 10 is desirably a vertical cylindrical structure having side walls 12 and a base 14. Steel reinforcing bars or rods (now shown) are embedded in the concrete.

Although the concrete structure is self-supporting and is resistant to low intensity external shocks, a large shock wave generated in close proximity to the structure 10 can substantially damage the structure, or even completely destroy it. In the embodiment of FIG. 1, a series of freeze pipes are driven or embedded into the earth around the structure 10. The freeze pipes 16, although not shown in detail in the drawings, are of a conventional freeze pipe construction. In such pipes, a coolant is passed downwardly in an annular space formed between an external pipe and an internal pipe (not shown) centrally located within the freeze pipe 16. The refrigerant or coolant passing downwardly in the freeze pipes 16, then passes inwardly through an opening (not shown) in the tubular central riser and passes upwardly therein. Thus, the coolant passing downwardly freezes the adjacent earth formation, including rock and soil. The coolant or refrigerant passes upwardly after use and the temperature thereof is reduced to the desired freezing temperature by any suitable means.

A header 18 is connected in parallel to the upper ends of the freeze pipes 16. Like the freeze pipes 16, the header 18 comprises inner and outer pipes, one such pipe being connected to the outer annular space of the freeze pipe 16 and the other such pipe being connected to the exhaust central pipe of the freeze pipes The coolant or refrigerant which passes downwardly in the plurality of spaced freeze pipes 16 freezes a zone, which completely surrounds the concrete structure 10. During the freezing of the zone around the concrete structure 10, the water in the concrete 10 itself becomes frozen, because of the proximity thereof to the freezing zone. Such freezing of the water in the concrete structure 10 thus further increases the shock load resistance of the belowground chamber. The total shock resistance of the concrete structure 10 is not only significantly enhanced by the provision of surrounding frozen zone 20, but it is also enhanced by the freezing of the moisture or water contained within the concrete structure 10 itself. Thus, the overall strength of the structure resulting from freezing the zone 20 is greater than the strength which would be attained by or expected from freezing alone or from the concrete structure alone, without any freezing.

Referring to FIG. 2, a preferred form of our invention is shown. In this embodiment, the frozen zone surrounding the concrete structure is frozen generally in the manner disclosed in detail in U.S. Pat. application Ser. No. 825,687 of William W. Bodle and Philip .I. Anderson, filed May I9, 1969 and now U.S. Pat. No. 3,646,765. Again, the excavation is made by use of conventional excavation techniques and a concrete structure 10, preferably reinforced, is provided, and is constructed around the excavation. A frozen zone 20 is provided, but is made in a manner distinct from that of the embodiment of FIG. 1.

Referring to FIG. 2, the concrete structure 10 is surrounded by an inner frozen zone 22 and an outer frozen zone 24. An intermediate initially unfrozen, enclosed or confined zone 26 is defined or formed between the inner zone 22 and the outer zone 24.

Both the inner zone and the outer zone are formed by the use of freeze pipes 28 and 30. These freeze pipes 28 and 30 are constructed in a manner similar to the freeze pipes 16 described with respect to the embodiment of FIG. 1, and refrigerant or a coolant, passing downwardly in the freeze pipes, freezes both the inner and outer zones 22 and 24. A header 32 interconnects the inner set of annular, spaced freeze pipes 28 and the outer set of freeze pipes 30 so that the refrigerant passes downwardly therein for freezing the earthen formation and upwardly after freezing the frozen zones. The inner zone 22 and the outer zone 24 are separated by the inner or confined unfrozen zone 26, the top and bottom portions of the zones 22 and 24 being interconnected by upper and lower frozen zones 34 and 36. The upper and lower zones 34 and 36 may be frozen, for example,.by freeze pipes, (now shown) which are insulated in the area of the central zone 26, which remains unfrozen while the zones 34 and 36 are frozen. These interconnecting zones 34 and 36 cooperate with the inner and outer frozen zones 22 and 24 to define the confined, unfrozen zone 26.

A pressure control pipe 38 extends into the unfrozen zone 26 and is interconnected to a water pressure control valve 40. By controlling the refrigerant flow in the freeze pipes 20 and 30, the originally unfrozen zone 26 undergoes controlled freezing. Because of the confined state of the unfrozen zone 26, the pressure in this zone increases as the water therein expands just prior to the attainment of freezing temperature. This expansion increases the pressure in the zone, by adjustment of the water pressure control valve 40, which communicates with the zone 26, through the pipe 38, to the desired pressure in the unfrozen zone. After the pressure desired for counteracting the various stresses created by a shock load has been attained in zone 26, the refrigeration or freezing of the unfrozen zone is controlled at a predetermined rate, by the use of the pressure control valve 40. The zone 26 is thus at a preselected internal compressive stress which greatly enhances the shock resisting stress of the entire structure, including the inner and outer zones 22 and 24, the intermediate zone 26, the freezing of the moisture in the concrete structure, and the concrete structure itself.

The required pressure in the unfrozen, high pressure zone may be calculated when the shock wave perimeter and the reinforced structure configuration and materials are known. This required pressure, in turn, establishes the configuration of and the extent of the frozen soil and rock zone surrounding the concrete structure, the refrigeration needs may then also be calculated. In one example, the materials in which a facility is constructed or fabricated are considered homogenous, isotropic, and elastic. The properties of such materials are considered constants, and the stress produced by an impact or input may be determined or assumed separately and algebraically added to the stress. The structure is assumed to respond linearly to the input. It is further assumed that plain strain conditions exist for the structure. In plain strain, sections of the structure do not move axially, the strain along the longitudinal axis of the cylinder being zero. The strains, therefore, are in the plains cutting the cylinder. More specifically, a cylindrical concrete structure is constructed vertically and relatively shallow. The medium in which the structure is constructed in soil is determined to have no effective strength in the unfrozen condition. For purposes of calculation, the hoop stress in the inner frozen ring, that is, the concrete structure 10, is assumed to be a function of the over pressure and the thermal stress, which are approximately equal in magnitude. When the hoop stress caused by an impact is 1658 psi, the frozen zone would be 70 feet thick to withstand such a stress. Low soil strength results in the large requirement for the wall thickness.

While in the foregoing, there has been provided a detailed description of particular embodiments of the present invention, it is to be understood that all equivalents obvious to those having skill in the art are to be included within the scope of the invention as claimed.

What we claim and desire to secure by Letters Patent 1. A belowground structure for resisting a shock wave passing through the zone in which said structure is located, said structure comprising a belowground chamber, a concrete wall surrounding said chamber, and a frozen earthen formation surrounding and adjacent said concrete wall, said concrete wall and said frozen earthen formation cooperating to provide a reinforced zone surrounding said chamber for resisting said shock wave.

2. The structure of claim 1 wherein said frozen zone comprises an inner frozen portion adjacent said concrete wall, an outer frozen portion spaced from said inner frozen portion, and an intermediate zone at least partially frozen under a preselected internal compressive stress.

3. The structure of claim 2 wherein the concrete wall has water therein which is frozen for enhancing the stress resistance of said structure.

4. The structure of claim 1 wherein said concrete wall is a steel reinforced, substantially vertical structure.

5. The structure of claim 1 wherein said concrete wall has water therein which is frozen for enhancing the stress resistance of said structure.

6. The structure of claim 1 wherein a top, and concrete bottom are provided, and said concrete wall is reinforced with steel.

7. A method for reinforcing a belowground chamber for resisting a shock wave passing through the zone in which said chamber is located, said method comprising steps of constructing a belowground chamber, constructing a concrete reinforcing wall around said chamber, and freezing the earth formation surrounding and adjacent said concrete wall, said earthen formation and said concrete wall cooperating to provide a reinforced zone surrounding said belowground chamber for resisting said shock wave.

8. The method of claim 7 wherein said freezing step comprises freezing a frozen zone around and adjacent said concrete wall, freezing a second zone around and spaced from said first zone to define a confined intermediate unfrozen zone intermediate said first and second frozen zone, and freezing at least a portion of said confined zone at a pressure above atmospheric pressure for providing a frozen intermediate zone at a prtaselected internal compressive stress.

he method of claim 7 wherein water in said concrete is frozen to enhance the stress resistant strength of said structure.

10. The method of claim 5 wherein water in said concrete is frozen to enhance the stress resistant strength of said structure.

11. The method of claim 7 wherein said concrete is reinforced with steel.

12. A method for reinforcing a pre-existing belowground chamber surrounded and reinforced by a concrete wall, said method comprising the steps of freezing a first portion around and adjacent said concrete wall, freezing a second zone around and spaced from said first zone to define a confined intermediate unfrozen zone intermediate said first and second zone, and freezing said confined and unfrozen zone at a pressure above atmospheric pressure for providing a frozen intermediate zone at a preselected internal compressive stress. 

1. A belowground structure for resisting a shock wave passing through the zone in which said structure is located, said structure comprising a belowground chamber, a concrete wall surrounding said chamber, and a frozen earthen formation surrounding and adjacent said concrete wall, said concrete wall and said frozen earthen formation cooperating to provide a reinforced zone surrounding said chamber for resisting said shock wave.
 1. A belowground structure for resisting a shock wave passing through the zone in which said structure is located, said structure comprising a belowground chamber, a concrete wall surrounding said chamber, and a frozen earthen formation surrounding and adjacent said concrete wall, said concrete wall and said frozen earthen formation cooperating to provide a reinforced zone surrounding said chamber for resisting said shock wave.
 2. The structure of claim 1 wherein said frozen zone comprises an inner frozen portion adjacent said concrete wall, an outer frozen portion spaced from said inner frozen portion, and an intermediate zone at least partially frozen under a preselected inTernal compressive stress.
 3. The structure of claim 2 wherein the concrete wall has water therein which is frozen for enhancing the stress resistance of said structure.
 4. The structure of claim 1 wherein said concrete wall is a steel reinforced, substantially vertical structure.
 5. The structure of claim 1 wherein said concrete wall has water therein which is frozen for enhancing the stress resistance of said structure.
 6. The structure of claim 1 wherein a top, and concrete bottom are provided, and said concrete wall is reinforced with steel.
 7. A method for reinforcing a belowground chamber for resisting a shock wave passing through the zone in which said chamber is located, said method comprising steps of constructing a belowground chamber, constructing a concrete reinforcing wall around said chamber, and freezing the earth formation surrounding and adjacent said concrete wall, said earthen formation and said concrete wall cooperating to provide a reinforced zone surrounding said belowground chamber for resisting said shock wave.
 8. The method of claim 7 wherein said freezing step comprises freezing a frozen zone around and adjacent said concrete wall, freezing a second zone around and spaced from said first zone to define a confined intermediate unfrozen zone intermediate said first and second frozen zone, and freezing at least a portion of said confined zone at a pressure above atmospheric pressure for providing a frozen intermediate zone at a preselected internal compressive stress.
 9. The method of claim 7 wherein water in said concrete is frozen to enhance the stress resistant strength of said structure.
 10. The method of claim 5 wherein water in said concrete is frozen to enhance the stress resistant strength of said structure.
 11. The method of claim 7 wherein said concrete is reinforced with steel. 